chore(ext/fleet): Delegate Kibana init to Elastic Agent

Ref. https://github.com/elastic/elastic-agent/blob/v8.5.0/internal/pkg/agent/cmd/container.go#L105-L116

docker-compose.yml

@@ -81,8 +81,6 @@ services:
       - "5601:5601"
     environment:
       KIBANA_SYSTEM_PASSWORD: ${KIBANA_SYSTEM_PASSWORD:-}
-      # Fleet plugin
-      KIBANA_FLEET_SETUP: '1'
     networks:
       - elk
     depends_on:

extensions/fleet/fleet-compose.yml

@@ -10,10 +10,12 @@ services:
       FLEET_SERVER_ENABLE: '1'
       FLEET_SERVER_INSECURE_HTTP: 'true'
       FLEET_SERVER_POLICY_ID: fleet-server-policy
+      # Fleet plugin in Kibana
+      KIBANA_FLEET_SETUP: '1'
       # Enrollment.
       # (a) Auto-enroll using basic authentication
-      KIBANA_FLEET_USERNAME: elastic
-      KIBANA_FLEET_PASSWORD: ${ELASTIC_PASSWORD:-}
+      ELASTICSEARCH_USERNAME: elastic
+      ELASTICSEARCH_PASSWORD: ${ELASTIC_PASSWORD:-}
       # (b) Enroll using a pre-generated service token
       #FLEET_SERVER_SERVICE_TOKEN: <service_token>
     ports: